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Adjust some comments about structure properties in pg_stat.h

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One comment of PgStat_TableCounts mentioned that its pending stats use
memcmp() to check for the all-zero case if there is any activity.  This
is not true since 07e9e28b56, as pg_memory_is_all_zeros() is used.

PgStat_FunctionCounts incorrectly documented that it relied on memcpy().
This has never been correct, and not relevant because function
statistics do not have an all-zero check for pending stats.

Checkpoint and bgwriter statistics have been always relying on memcmp()
or pg_memory_is_all_zeros() (since 07e9e28b56 for the latter), and never
mentioned the dependency on event counters for their all-zero checks.
Let's document these properties, like the table statistics.

Author: Bertrand Drouvot
Discussion: https://postgr.es/m/Z1hNLvcPgVLPxCoc@ip-10-97-1-34.eu-west-3.compute.internal
This commit is contained in:
Michael Paquier 2024-12-12 16:59:22 +09:00
parent bd10ec5297
commit 4766438aa3
1 changed files with 20 additions and 5 deletions
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25
src/include/pgstat.h
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@ -128,9 +128,6 @@ typedef int64 PgStat_Counter;
/* ---------- /* ----------
* PgStat_FunctionCounts The actual per-function counts kept by a backend * PgStat_FunctionCounts The actual per-function counts kept by a backend
* *
* This struct should contain only actual event counters, because we memcmp
* it against zeroes to detect whether there are any pending stats.
*
* Note that the time counters are in instr_time format here. We convert to * Note that the time counters are in instr_time format here. We convert to
* microseconds in PgStat_Counter format when flushing out pending statistics. * microseconds in PgStat_Counter format when flushing out pending statistics.
* ---------- * ----------
@ -172,8 +169,10 @@ typedef struct PgStat_BackendSubEntry
/* ---------- /* ----------
* PgStat_TableCounts The actual per-table counts kept by a backend * PgStat_TableCounts The actual per-table counts kept by a backend
* *
* This struct should contain only actual event counters, because we memcmp * This struct should contain only actual event counters, because we make use
* it against zeroes to detect whether there are any stats updates to apply. * of pg_memory_is_all_zeros() to detect whether there are any stats updates
* to apply.
*
* It is a component of PgStat_TableStatus (within-backend state). * It is a component of PgStat_TableStatus (within-backend state).
* *
* Note: for a table, tuples_returned is the number of tuples successfully * Note: for a table, tuples_returned is the number of tuples successfully
@ -282,6 +281,14 @@ typedef struct PgStat_ArchiverStats
TimestampTz stat_reset_timestamp; TimestampTz stat_reset_timestamp;
} PgStat_ArchiverStats; } PgStat_ArchiverStats;
/* ---------
* PgStat_BgWriterStats Background Writer statistics
*
* This struct should contain only actual event counters, because we make use
* of pg_memory_is_all_zeros() to detect whether there are any stats updates
* to apply.
* ---------
*/
typedef struct PgStat_BgWriterStats typedef struct PgStat_BgWriterStats
{ {
PgStat_Counter buf_written_clean; PgStat_Counter buf_written_clean;
@ -290,6 +297,14 @@ typedef struct PgStat_BgWriterStats
TimestampTz stat_reset_timestamp; TimestampTz stat_reset_timestamp;
} PgStat_BgWriterStats; } PgStat_BgWriterStats;
/* --------
* PgStat_CheckpointerStats Checkpoint statistics
*
* This struct should contain only actual event counters, because we make use
* of pg_memory_is_all_zeros() to detect whether there are any stats updates to
* apply.
* ---------
*/
typedef struct PgStat_CheckpointerStats typedef struct PgStat_CheckpointerStats
{ {
PgStat_Counter num_timed; PgStat_Counter num_timed;